NAME

tar - format of tape archive files

DESCRIPTION

The tar archive format collects any number of files, directories, and
other file system objects (symbolic links, device nodes, etc.) into a
single stream of bytes. The format was originally designed to be used
with tape drives that operate with fixed-size blocks, but is widely used
as a general packaging mechanism.
GeneralFormat
A tar archive consists of a series of 512-byte records. Each file system
object requires a header record which stores basic metadata (pathname,
owner, permissions, etc.) and zero or more records containing any file
data. The end of the archive is indicated by two records consisting
entirely of zero bytes.
For compatibility with tape drives that use fixed block sizes, programs
that read or write tar files always read or write a fixed number of
records with each I/O operation. These “blocks” are always a multiple of
the record size. The maximum block size supported by early
implementations was 10240 bytes or 20 records. This is still the default
for most implementations although block sizes of 1MiB (2048 records) or
larger are commonly used with modern high-speed tape drives. (Note: the
terms “block” and “record” here are not entirely standard; this document
follows the convention established by John Gilmore in documenting pdtar.)
Old-StyleArchiveFormat
The original tar archive format has been extended many times to include
additional information that various implementors found necessary. This
section describes the variant implemented by the tar command included in
Version 7 AT&T UNIX, which seems to be the earliest widely-used version
of the tar program.
The header record for an old-style tar archive consists of the following:
struct header_old_tar {
char name[100];
char mode[8];
char uid[8];
char gid[8];
char size[12];
char mtime[12];
char checksum[8];
char linkflag[1];
char linkname[100];
char pad[255];
};
All unused bytes in the header record are filled with nulls.
name Pathname, stored as a null-terminated string. Early tar
implementations only stored regular files (including hardlinks to
those files). One common early convention used a trailing "/"
character to indicate a directory name, allowing directory
permissions and owner information to be archived and restored.
mode File mode, stored as an octal number in ASCII.
uid, gid
User id and group id of owner, as octal numbers in ASCII.
size Size of file, as octal number in ASCII. For regular files only,
this indicates the amount of data that follows the header. In
particular, this field was ignored by early tar implementations
when extracting hardlinks. Modern writers should always store a
zero length for hardlink entries.
mtime Modification time of file, as an octal number in ASCII. This
indicates the number of seconds since the start of the epoch,
00:00:00 UTC January 1, 1970. Note that negative values should
be avoided here, as they are handled inconsistently.
checksum
Header checksum, stored as an octal number in ASCII. To compute
the checksum, set the checksum field to all spaces, then sum all
bytes in the header using unsigned arithmetic. This field should
be stored as six octal digits followed by a null and a space
character. Note that many early implementations of tar used
signed arithmetic for the checksum field, which can cause
interoperability problems when transferring archives between
systems. Modern robust readers compute the checksum both ways
and accept the header if either computation matches.
linkflag, linkname
In order to preserve hardlinks and conserve tape, a file with
multiple links is only written to the archive the first time it
is encountered. The next time it is encountered, the linkflag is
set to an ASCII ‘1’ and the linkname field holds the first name
under which this file appears. (Note that regular files have a
null value in the linkflag field.)
Early tar implementations varied in how they terminated these fields.
The tar command in Version 7 AT&T UNIX used the following conventions
(this is also documented in early BSD manpages): the pathname must be
null-terminated; the mode, uid, and gid fields must end in a space and a
null byte; the size and mtime fields must end in a space; the checksum is
terminated by a null and a space. Early implementations filled the
numeric fields with leading spaces. This seems to have been common
practice until the IEEE Std 1003.1-1988 (“POSIX.1”) standard was
released. For best portability, modern implementations should fill the
numeric fields with leading zeros.
Pre-POSIXArchives
An early draft of IEEE Std 1003.1-1988 (“POSIX.1”) served as the basis
for John Gilmore’s pdtar program and many system implementations from the
late 1980s and early 1990s. These archives generally follow the POSIX
ustar format described below with the following variations:
· The magic value is “ustar ” (note the following space). The
version field contains a space character followed by a null.
· The numeric fields are generally filled with leading spaces (not
leading zeros as recommended in the final standard).
· The prefix field is often not used, limiting pathnames to the 100
characters of old-style archives.
POSIXustarArchives
IEEE Std 1003.1-1988 (“POSIX.1”) defined a standard tar file format to be
read and written by compliant implementations of tar(1). This format is
often called the “ustar” format, after the magic value used in the
header. (The name is an acronym for “Unix Standard TAR”.) It extends
the historic format with new fields:
struct header_posix_ustar {
char name[100];
char mode[8];
char uid[8];
char gid[8];
char size[12];
char mtime[12];
char checksum[8];
char typeflag[1];
char linkname[100];
char magic[6];
char version[2];
char uname[32];
char gname[32];
char devmajor[8];
char devminor[8];
char prefix[155];
char pad[12];
};
typeflag
Type of entry. POSIX extended the earlier linkflag field with
several new type values:
“0” Regular file. NUL should be treated as a synonym, for
compatibility purposes.
“1” Hard link.
“2” Symbolic link.
“3” Character device node.
“4” Block device node.
“5” Directory.
“6” FIFO node.
“7” Reserved.
Other A POSIX-compliant implementation must treat any
unrecognized typeflag value as a regular file. In
particular, writers should ensure that all entries have a
valid filename so that they can be restored by readers
that do not support the corresponding extension.
Uppercase letters "A" through "Z" are reserved for custom
extensions. Note that sockets and whiteout entries are
not archivable.
It is worth noting that the size field, in particular, has
different meanings depending on the type. For regular files, of
course, it indicates the amount of data following the header.
For directories, it may be used to indicate the total size of all
files in the directory, for use by operating systems that pre-
allocate directory space. For all other types, it should be set
to zero by writers and ignored by readers.
magic Contains the magic value “ustar” followed by a NUL byte to
indicate that this is a POSIX standard archive. Full compliance
requires the uname and gname fields be properly set.
version
Version. This should be “00” (two copies of the ASCII digit
zero) for POSIX standard archives.
uname, gname
User and group names, as null-terminated ASCII strings. These
should be used in preference to the uid/gid values when they are
set and the corresponding names exist on the system.
devmajor, devminor
Major and minor numbers for character device or block device
entry.
name, prefix
If the pathname is too long to fit in the 100 bytes provided by
the standard format, it can be split at any / character with the
first portion going into the prefix field. If the prefix field
is not empty, the reader will prepend the prefix value and a /
character to the regular name field to obtain the full pathname.
The standard does not require a trailing / character on directory
names, though most implementations still include this for
compatibility reasons.
Note that all unused bytes must be set to NUL.
Field termination is specified slightly differently by POSIX than by
previous implementations. The magic, uname, and gname fields must have a
trailing NUL. The pathname, linkname, and prefix fields must have a
trailing NUL unless they fill the entire field. (In particular, it is
possible to store a 256-character pathname if it happens to have a / as
the 156th character.) POSIX requires numeric fields to be zero-padded in
the front, and requires them to be terminated with either space or NUL
characters.
Currently, most tar implementations comply with the ustar format,
occasionally extending it by adding new fields to the blank area at the
end of the header record.
PaxInterchangeFormat
There are many attributes that cannot be portably stored in a POSIX ustar
archive. IEEE Std 1003.1-2001 (“POSIX.1”) defined a “pax interchange
format” that uses two new types of entries to hold text-formatted
metadata that applies to following entries. Note that a pax interchange
format archive is a ustar archive in every respect. The new data is
stored in ustar-compatible archive entries that use the “x” or “g”
typeflag. In particular, older implementations that do not fully support
these extensions will extract the metadata into regular files, where the
metadata can be examined as necessary.
An entry in a pax interchange format archive consists of one or two
standard ustar entries, each with its own header and data. The first
optional entry stores the extended attributes for the following entry.
This optional first entry has an "x" typeflag and a size field that
indicates the total size of the extended attributes. The extended
attributes themselves are stored as a series of text-format lines encoded
in the portable UTF-8 encoding. Each line consists of a decimal number,
a space, a key string, an equals sign, a value string, and a new line.
The decimal number indicates the length of the entire line, including the
initial length field and the trailing newline. An example of such a
field is:
25 ctime=1084839148.1212\n
Keys in all lowercase are standard keys. Vendors can add their own keys
by prefixing them with an all uppercase vendor name and a period. Note
that, unlike the historic header, numeric values are stored using
decimal, not octal. A description of some common keys follows:
atime, ctime, mtime
File access, inode change, and modification times. These fields
can be negative or include a decimal point and a fractional
value.
uname, uid, gname, gid
User name, group name, and numeric UID and GID values. The user
name and group name stored here are encoded in UTF8 and can thus
include non-ASCII characters. The UID and GID fields can be of
arbitrary length.
linkpath
The full path of the linked-to file. Note that this is encoded
in UTF8 and can thus include non-ASCII characters.
path The full pathname of the entry. Note that this is encoded in
UTF8 and can thus include non-ASCII characters.
realtime.*, security.*
These keys are reserved and may be used for future
standardization.
size The size of the file. Note that there is no length limit on this
field, allowing conforming archives to store files much larger
than the historic 8GB limit.
SCHILY.*
Vendor-specific attributes used by Joerg Schilling’s star
implementation.
SCHILY.acl.access, SCHILY.acl.default
Stores the access and default ACLs as textual strings in a format
that is an extension of the format specified by POSIX.1e draft
17. In particular, each user or group access specification can
include a fourth colon-separated field with the numeric UID or
GID. This allows ACLs to be restored on systems that may not
have complete user or group information available (such as when
NIS/YP or LDAP services are temporarily unavailable).
SCHILY.devminor, SCHILY.devmajor
The full minor and major numbers for device nodes.
SCHILY.fflags
The file flags.
SCHILY.realsize
The full size of the file on disk. XXX explain? XXX
SCHILY.dev,SCHILY.ino, SCHILY.nlinks
The device number, inode number, and link count for the entry.
In particular, note that a pax interchange format archive using
Joerg Schilling’s SCHILY.* extensions can store all of the data
from structstat.
LIBARCHIVE.xattr.namespace.key
Libarchive stores POSIX.1e-style extended attributes using keys
of this form. The key value is URL-encoded: All non-ASCII
characters and the two special characters “=” and “%” are encoded
as “%” followed by two uppercase hexadecimal digits. The value
of this key is the extended attribute value encoded in base 64.
XXX Detail the base-64 format here XXX
VENDOR.*
XXX document other vendor-specific extensions XXX
Any values stored in an extended attribute override the corresponding
values in the regular tar header. Note that compliant readers should
ignore the regular fields when they are overridden. This is important,
as existing archivers are known to store non-compliant values in the
standard header fields in this situation. There are no limits on length
for any of these fields. In particular, numeric fields can be
arbitrarily large. All text fields are encoded in UTF8. Compliant
writers should store only portable 7-bit ASCII characters in the standard
ustar header and use extended attributes whenever a text value contains
non-ASCII characters.
In addition to the x entry described above, the pax interchange format
also supports a g entry. The g entry is identical in format, but
specifies attributes that serve as defaults for all subsequent archive
entries. The g entry is not widely used.
Besides the new x and g entries, the pax interchange format has a few
other minor variations from the earlier ustar format. The most troubling
one is that hardlinks are permitted to have data following them. This
allows readers to restore any hardlink to a file without having to rewind
the archive to find an earlier entry. However, it creates complications
for robust readers, as it is no longer clear whether or not they should
ignore the size field for hardlink entries.
GNUTarArchives
The GNU tar program started with a pre-POSIX format similar to that
described earlier and has extended it using several different mechanisms:
It added new fields to the empty space in the header (some of which was
later used by POSIX for conflicting purposes); it allowed the header to
be continued over multiple records; and it defined new entries that
modify following entries (similar in principle to the x entry described
above, but each GNU special entry is single-purpose, unlike the general-
purpose x entry). As a result, GNU tar archives are not POSIX
compatible, although more lenient POSIX-compliant readers can
successfully extract most GNU tar archives.
struct header_gnu_tar {
char name[100];
char mode[8];
char uid[8];
char gid[8];
char size[12];
char mtime[12];
char checksum[8];
char typeflag[1];
char linkname[100];
char magic[6];
char version[2];
char uname[32];
char gname[32];
char devmajor[8];
char devminor[8];
char atime[12];
char ctime[12];
char offset[12];
char longnames[4];
char unused[1];
struct {
char offset[12];
char numbytes[12];
} sparse[4];
char isextended[1];
char realsize[12];
char pad[17];
};
typeflag
GNU tar uses the following special entry types, in addition to
those defined by POSIX:
7 GNU tar treats type "7" records identically to type "0"
records, except on one obscure RTOS where they are used
to indicate the pre-allocation of a contiguous file on
disk.
D This indicates a directory entry. Unlike the POSIX-
standard "5" typeflag, the header is followed by data
records listing the names of files in this directory.
Each name is preceded by an ASCII "Y" if the file is
stored in this archive or "N" if the file is not stored
in this archive. Each name is terminated with a null,
and an extra null marks the end of the name list. The
purpose of this entry is to support incremental backups;
a program restoring from such an archive may wish to
delete files on disk that did not exist in the directory
when the archive was made.
Note that the "D" typeflag specifically violates POSIX,
which requires that unrecognized typeflags be restored as
normal files. In this case, restoring the "D" entry as a
file could interfere with subsequent creation of the
like-named directory.
K The data for this entry is a long linkname for the
following regular entry.
L The data for this entry is a long pathname for the
following regular entry.
M This is a continuation of the last file on the previous
volume. GNU multi-volume archives guarantee that each
volume begins with a valid entry header. To ensure this,
a file may be split, with part stored at the end of one
volume, and part stored at the beginning of the next
volume. The "M" typeflag indicates that this entry
continues an existing file. Such entries can only occur
as the first or second entry in an archive (the latter
only if the first entry is a volume label). The size
field specifies the size of this entry. The offset field
at bytes 369-380 specifies the offset where this file
fragment begins. The realsize field specifies the total
size of the file (which must equal size plus offset).
When extracting, GNU tar checks that the header file name
is the one it is expecting, that the header offset is in
the correct sequence, and that the sum of offset and size
is equal to realsize.
N Type "N" records are no longer generated by GNU tar.
They contained a list of files to be renamed or symlinked
after extraction; this was originally used to support
long names. The contents of this record are a text
description of the operations to be done, in the form
“Rename %s to %s\n” or “Symlink %s to %s\n”; in either
case, both filenames are escaped using K&R C syntax. Due
to security concerns, "N" records are now generally
ignored when reading archives.
S This is a “sparse” regular file. Sparse files are stored
as a series of fragments. The header contains a list of
fragment offset/length pairs. If more than four such
entries are required, the header is extended as necessary
with “extra” header extensions (an older format that is
no longer used), or “sparse” extensions.
V The name field should be interpreted as a tape/volume
header name. This entry should generally be ignored on
extraction.
magic The magic field holds the five characters “ustar” followed by a
space. Note that POSIX ustar archives have a trailing null.
version
The version field holds a space character followed by a null.
Note that POSIX ustar archives use two copies of the ASCII digit
“0”.
atime, ctime
The time the file was last accessed and the time of last change
of file information, stored in octal as with mtime.
longnames
This field is apparently no longer used.
Sparse offset/numbytes
Each such structure specifies a single fragment of a sparse file.
The two fields store values as octal numbers. The fragments are
each padded to a multiple of 512 bytes in the archive. On
extraction, the list of fragments is collected from the header
(including any extension headers), and the data is then read and
written to the file at appropriate offsets.
isextended
If this is set to non-zero, the header will be followed by
additional “sparse header” records. Each such record contains
information about as many as 21 additional sparse blocks as shown
here:
struct gnu_sparse_header {
struct {
char offset[12];
char numbytes[12];
} sparse[21];
char isextended[1];
char padding[7];
};
realsize
A binary representation of the file’s complete size, with a much
larger range than the POSIX file size. In particular, with M
type files, the current entry is only a portion of the file. In
that case, the POSIX size field will indicate the size of this
entry; the realsize field will indicate the total size of the
file.
GNUtarpaxarchives
GNU tar 1.14 (XXX check this XXX) and later will write pax interchange
format archives when you specify the --posix flag. This format uses
custom keywords to store sparse file information. There have been three
iterations of this support, referred to as “0.0”, “0.1”, and “1.0”.
GNU.sparse.numblocks, GNU.sparse.offset, GNU.sparse.numbytes,
GNU.sparse.size
The “0.0” format used an initial GNU.sparse.numblocks attribute
to indicate the number of blocks in the file, a pair of
GNU.sparse.offset and GNU.sparse.numbytes to indicate the offset
and size of each block, and a single GNU.sparse.size to indicate
the full size of the file. This is not the same as the size in
the tar header because the latter value does not include the size
of any holes. This format required that the order of attributes
be preserved and relied on readers accepting multiple appearances
of the same attribute names, which is not officially permitted by
the standards.
GNU.sparse.map
The “0.1” format used a single attribute that stored a comma-
separated list of decimal numbers. Each pair of numbers
indicated the offset and size, respectively, of a block of data.
This does not work well if the archive is extracted by an
archiver that does not recognize this extension, since many pax
implementations simply discard unrecognized attributes.
GNU.sparse.major, GNU.sparse.minor, GNU.sparse.name, GNU.sparse.realsize
The “1.0” format stores the sparse block map in one or more
512-byte blocks prepended to the file data in the entry body.
The pax attributes indicate the existence of this map (via the
GNU.sparse.major and GNU.sparse.minor fields) and the full size
of the file. The GNU.sparse.name holds the true name of the
file. To avoid confusion, the name stored in the regular tar
header is a modified name so that extraction errors will be
apparent to users.
SolarisTar
XXX More Details Needed XXX
Solaris tar (beginning with SunOS XXX 5.7 ?? XXX) supports an “extended”
format that is fundamentally similar to pax interchange format, with the
following differences:
· Extended attributes are stored in an entry whose type is X, not
x, as used by pax interchange format. The detailed format of
this entry appears to be the same as detailed above for the x
entry.
· An additional A entry is used to store an ACL for the following
regular entry. The body of this entry contains a seven-digit
octal number followed by a zero byte, followed by the textual ACL
description. The octal value is the number of ACL entries plus a
constant that indicates the ACL type: 01000000 for POSIX.1e ACLs
and 03000000 for NFSv4 ACLs.
AIXTar
XXX More details needed XXX
MacOSXTar
The tar distributed with Apple’s Mac OS X stores most regular files as
two separate entries in the tar archive. The two entries have the same
name except that the first one has “._” added to the beginning of the
name. This first entry stores the “resource fork” with additional
attributes for the file. The Mac OS X CopyFile() API is used to separate
a file on disk into separate resource and data streams and to reassemble
those separate streams when the file is restored to disk.
OtherExtensions
One obvious extension to increase the size of files is to eliminate the
terminating characters from the various numeric fields. For example, the
standard only allows the size field to contain 11 octal digits, reserving
the twelfth byte for a trailing NUL character. Allowing 12 octal digits
allows file sizes up to 64 GB.
Another extension, utilized by GNU tar, star, and other newer tar
implementations, permits binary numbers in the standard numeric fields.
This is flagged by setting the high bit of the first byte. This permits
95-bit values for the length and time fields and 63-bit values for the
uid, gid, and device numbers. GNU tar supports this extension for the
length, mtime, ctime, and atime fields. Joerg Schilling’s star program
supports this extension for all numeric fields. Note that this extension
is largely obsoleted by the extended attribute record provided by the pax
interchange format.
Another early GNU extension allowed base-64 values rather than octal.
This extension was short-lived and is no longer supported by any
implementation.

SEEALSO

STANDARDS

The tar utility is no longer a part of POSIX or the Single Unix Standard.
It last appeared in Version 2 of the Single UNIX Specification (“SUSv2”).
It has been supplanted in subsequent standards by pax(1). The ustar
format is currently part of the specification for the pax(1) utility.
The pax interchange file format is new with IEEE Std 1003.1-2001
(“POSIX.1”).

HISTORY

A tar command appeared in Seventh Edition Unix, which was released in
January, 1979. It replaced the tp program from Fourth Edition Unix which
in turn replaced the tap program from First Edition Unix. John Gilmore’s
pdtar public-domain implementation (circa 1987) was highly influential
and formed the basis of GNUtar (circa 1988). Joerg Shilling’s star
archiver is another open-source (GPL) archiver (originally developed
circa 1985) which features complete support for pax interchange format.
This documentation was written as part of the libarchive and bsdtar
project by Tim Kientzle 〈kientzle@FreeBSD.org〉.